Fault tolerant capability (that is, the ability to continue to operate, at full or reduced capacity, in the presence of a fault within one or more elements) is often required in generator or starter/generator systems used in applications requiring a high level of reliability. A typical generator system 1, shown schematically in FIG. 1, includes a machine (e.g. a permanent magnet alternator or PMA) 2 coupled to a power controller (e.g. power conversion unit or PCU) 3, which provides DC output power at terminals 5. Fault tolerant subsystems may be implemented by configuring one or more functions within the machine and/or the power controller with redundant or multiple elements. Fault tolerant system architectures, including use of multiple isolated windings, incorporating redundancy in the machine and/or the power controller/converter functions, etc. are described in more detail in U.S. Pat. No. 7,064,526 (assigned to the same assignee as the present disclosure), the entire disclosure of which is incorporated herein by reference.
When one of these elements is disabled, such as an open winding fault within a single phase of the machine stator, the generator or starter/generator function is then provided using the redundant element. When operating in this fault mode, the machine loads are typically unbalanced, which can result in significant levels of torque ripple. The high levels of torque ripple can cause damage to the machine, its drive system and/or the prime mover.
Accordingly, there is a need for a fault tolerant generator or starter/generator system which exhibits low torque ripple when operating in a fault mode.